1. Field of the Invention
The present invention relates to a system for monitoring carriers of peripheral zones, more particularly relates to a system for monitoring carriers of peripheral zones in a mobile telecommunications system where an individual mobile unit communicates with several base stations.
2. Description of the Related Art
In recent years, there has been striking progress in car phones, cellular phones, and other digital mobile telecommunications systems. One of the themes of development has been the reduction of the radius of the service zones (cells). In general, a digital mobile telecommunications system is comprised by a radio communication network consisting of a close-knit honeycomb arrangement of plurality of grids of zones having exactly the same configuration, i.e., a configuration of a plurality of zones, for example seven, to which differing carrier frequencies have been allocated. To improve the efficiency of reuse of the carrier frequencies by the grids of zones, the trend in recent years, as mentioned earlier, has been to reduce the radius of the individual zones.
If the zone radius is reduced in this way, however, a mobile unit may keep on changing the zone it is engaged with, so the frequency of channel switching and zone shifting ends up increasing. Here, "channel switching" refers to the operation for switching the speech channel of a mobile unit from one of a base station in an engaged zone to one of the base station of another zone when a mobile unit involved in a call moves from one zone to an adjoining peripheral zone. The smaller the zone radius, the more frequent the channel switching. Further, "zone shifting" refers to the operation for switching the zone for line connection from one zone to another when a mobile unit in a radio communication network moves from its current line connection zone to an adjoining zone.
By reducing the zone radius, the base stations end up with a considerably heavier load in controlling the channel switching. Therefore, the practice has been to lighten the load by placing part of it on the mobile unit. That is, a mobile unit uses its unused slots (idle time) to monitor the carriers received from the peripheral zones around it and sends monitoring information to the engaged base station. The base station receiving this uses the information to determine to which zone the base station which the mobile unit should communicate with belongs and, if necessary, switches the channel from the engaged zone to an adjoining peripheral zone.
Since the base station switches channels using the monitoring information notified to it from the mobile unit under time division multiple access (TDMA) control, it is necessary that the mobile unit give the base station accurate monitoring information about the peripheral zones. If a mobile unit does not select the correct zone for line connection (3), it cannot originate or receive a call, so has to accurately recognize if the zone it shifts to is correct.
As will be explained in detail later referring to the figures, a mobile unit detects the received field strengths of the transmission carriers received at each idle time and, if engaged in a call, sends those values periodically to the nearest base station. When the level of reception from a peripheral base station becomes higher in the mobile unit than the level of reception from that original engaged base station, the original engaged base station switches the channel to one of the zone with the highest level of received field strength from the viewpoint of the mobile unit. When the mobile unit is in a waiting state where it is standing by to receive or send a call, it shifts the zone it is engaged with to that of the base station sending the carrier with the highest received field strength which it receives. The mobile unit subsequently can engage in normal communication.
That is, the base station checks that the level of the received field strength of the transmission carrier received by the mobile unit is the highest and switches the channel to one of the zone sending the transmission carrier. On the other hand, the mobile unit shifts the zone it is engaged with to the zone with the highest level of received field strength.
Assume here that there is a source of interference. Assume further that it radiates a relatively high level wave and that the frequency of the radiated wave is substantially equal to the frequency of a carrier sent from the engaged base station.
In such a situation, due to the reasons discussed later, the mobile unit sometimes cannot maintain communication with its current base station from which it receives a wave at a suitable received field strength.
On the other hand, when there are a number of other mobile unit close to the source of interference, these mobile units also try to switch channels or shift zones to the other base station with the same frequency, so there is the problem that the normal connection operation of that base station is disturbed.
Further, a mobile unit in a waiting state would receive the strong interference and switch channels to the same, but since that channel does not contain any data, it would then switch to the channel with the next highest received level. Since it again receives the interference, however, it again switches to that channel. While switching to that channel, it cannot originate or terminate calls, which poses a problem in system operation. The same applies to the other mobile units.
Note that the above problems may also occur due to factors other than a source of interference (explained later).